1. Field of the Invention
The present invention relates to a photomask (a reticle) for use in a lithography step of a semiconductor device, a photomask superimposition correcting method, and a manufacturing method of a semiconductor device using the photomask.
2. Description of the Related Art
A photomask for use in a lithography step of a semiconductor device includes an alignment mark and a superimposition inspection mark.
The alignment mark is used for alignment in correctly superimposing, on a device pattern (a circuit pattern) already formed on a semiconductor substrate, a device pattern which is formed on the photomask and which is to be next formed on the substrate to expose the pattern. The alignment is performed in a state in which the photomask is set on an exposure apparatus.
The superimposition inspection mark is used in superimposition inspection to inspect whether or not the device pattern formed by the exposure has been exposed at a correct position. The superimposition inspection is performed using a superimposition inspection apparatus.
These alignment mark and superimposition inspection mark are usually larger than the device pattern, and have different pattern shapes so that the marks can easily be detected by the exposure apparatus and the superimposition inspection apparatus, respectively.
In recent years, with miniaturization of the device pattern of the semiconductor device, the sizes and the pattern shapes of the alignment mark and the superimposition inspection mark have been greatly different from those of the device pattern. Therefore, an alignment error between these patterns also increases.
Heretofore, as means for eliminating the alignment error produced by the large difference in size and pattern shape, there have been suggested a method for reducing the length of the alignment mark to a length close to that of a circuit pattern shape (e.g., Jpn. Pat. Appln. KOKAI Publication No. 9-102457 [Patent Document 1]) and a method for forming the mark by use of a part of the device pattern, or a similar pattern (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-64055 [Patent Document 2]).
However, in these methods, a certain effect can be anticipated with respect to a difference of optimum exposure, but deviation of a transfer position due to an aberration of a projection lens, and an alignment error due to a measurement error caused by pattern asymmetry cannot be eliminated, and lowering of a superimposition inspection precision due to the error cannot be suppressed.